Aircraft Yaw...

[A320_Murray]

Hello,
Recently I had the chance to have an hours flight in a Cessna 172. Whilst doing the usual routine i.e use of the controls surface, what effect they have on the aircraft path etc, I found myself wondering why the instructor was using backforce on the yoke whilst I was applying rudder.
My question is why? Why did he use backforce. Obviously I assumed that whilst using rudder (no other control surfaces) the nose would drop, Why does it do this? If that is indeed what is does, it must! I did ask him, but his reply was "We haven't got enough time to explain" hmm...
Well I am sure you great guys will be able to help me...

Regards,
Matthew

[Mad (Flt) Scientist]

The instructor was presumably pulling back on the yoke to maintain your airspeed. There are two possible reasons for that:

1. The movement of the rudder had affected the flow over the whole empennage, including the tailplane and elevator, and therefore the hinge moment characteristics of the elevator. The aircraft having manual controls, that might change the force required to hold the elevator in a fixed position. So it would go from no force (with no rudder applied) to some force (with rudder applied). In this case the yoke itself is not moving, but the force changes. I consider this the least likely explanation

2. The rudder movement has induced a pitching moment on the aircraft (either directly or indirectly, see below) which the instructor is counteracting by appling a slightly different elevator angle to maintain the aircraft in trim. In this case the yoke would move in addition to a force being required. The pitching moment may be induced two ways (or some combination of the two):

(a) Directly induced pitching moment. In this case the movement of the rudder creates a pitching moment directly, even if nothing else were to change on the aircraft. This may be due to the location of the forces created by the rudder (side force on the fin and drag on the fin). While the sideforce is unlikely to create a pitching moment (on a single fin aircraft such as this) the drag is unlikely to be acting directly through the aircraft centre of gravity. This will create a pitching moment due to the drag. I consider this unlikely, as it would be more likely to cause a nose-up moment, which would require forward yoke. It is also possible that the flow disturbance of the rudder's movement has created asymmetric flow over the tailplane, which may be creating a pitching moment change. Again, I feel this would be a secondary effect

(b) Indirectly induced pitching moment. In this case it is not the rudder which causes the pitching moment, but the rudder causes something else to change, which is then the source of the pitching moment. If the aircraft were fixed (as in a wind tunnel) there would be no pitching moment here. But with it free to respond to the rudder, there can be. I consider this the most likely explanation What is probably happening is that with application of rudder there is sideslip roduced on the aircraft. It is common for aircraft to have a sensitivity of pitching behaviour to sideslip, due to, among other things, the change in the wing lift as the sweep of the wing is varied relative to the flow.

Hope that helps a bit. Sorry for the "maybes" rather than the definitive answer you were expecting.

[Earthmover]

Well Matthew, you'll see from the text-books that the secondary effect of yaw is roll, followed by the nose nodding down and, if left unattended, a spiral dive.

But, if I may say so, you are asking the wrong question. The real query should be "Why does this instructor have no time to explain this to me?" Arch one eyebrow, see how things go ..... and if it happens again, find a proper one who has a schedule that allows a de-brief! Er, he did brief you beforehand ... didn't he?

[A320_Murray]

Hello,
Thanks Mad (Flt) Scientist and Earthmover. I amn't going through my PPL (yet), I was up on a one-off flight (sorta thing) and was just curious as to why. Thanks very much, didn't know there was more than one way for rudder induced pitch. Thanks again guys :)
Regards,
Matthew

[FlyingForFun]

Well done for noticing this - I know I didn't notice it during my trial lesson, and as my flying advanced I started learning about this kind of effect, and wondering why I didn't see the nose dropping when playing with the rudder on my trial lesson! :)

I'm not in a position to say that Mad Scientist is wrong, but I'd always thought that it was much simpler than he makes out. Yaw induces roll - the reason for this is that the outside wing is going faster, and so produces more lift. After rolling, the aircraft is banked, so the lift from the wings is no longer directed upwards - there's only an element of it which is pointing upwards. Therefore, you need to increase the lift in order to avoid loosing height. The normal way of doing this is to apply back pressure.

Or, alternatively, your instructor may also have been using ailerons to prevent the aircraft from rolling. In this case, the lift would still be directed upwards because the aircraft wouldn't be banked. However, you'd now be skidding through the air - the aircraft is not as streamlined as it is in normal, balanced flight. This would add extra drag, and slow the aircraft down - and the reduced speed woul reduce the amount of lift the wings produce. Again, this can be compensated for by back-pressure.

I don't think Mad Scientist is right when he says: "The instructor was presumably pulling back on the yoke to maintain your airspeed". I suspect the airspeed was lower than in straight+level flight while you were doing these exercises, and the instructor was actually maintaining altitude.

Did your instructor really not have time to explain? Or did he prefer to talk about this later on when you look at secondary effects of controls in more detail?

Anyway, glad you enjoyed the experience!

FFF
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[A320_Murray]

Hello,
Thanks FlyingForFun. He was controlling the yoke and I was controlling the rudder. I noticed as I was applying left rudder (i.e turing the nose to the left) he was appyling a right bank, I knew that the rudder would start a bank also, so I didn't include that. I was just a bit curious why he was using back pressure because the aircraft was trimmed for level flight and he corrected the bank with right on the yoke...:)
Oh dear hehehehe...he said also 2It would take as long as this lesson to explain it ont he ground" hmm....wierd...:rolleyes:

[Who has control?]

'Yaw causes Roll and Roll causes Yaw.'

One of the few things I remember from my first lesson.

[Croqueteer]

Right rudder, yaw right, roll right wing low, rudder vector now acting as down elevator, therefore up elevator to compensate. Similarly for left rudder.

[foghorn]

Now out of all of the explanations on offer here, I agree most with FFF's second explanation:

Flying cross controls is inefficient giving additional drag
Therefore the airspeed decreases from that when properly balanced
Therefore the aircraft wants to descend
So raise the nose slightly to maintain level flight

Cheers!
foggy.

[A320_Murray]

Hey,
foghorn - that makes sense to me....a logical answer...thank-you very much :)

[Croqueteer]

Try again. The nose wants to drop before the speed has had time to fall. Fly it and observe it.

[foghorn]

Croqueteer - we might be talking apples and oranges here - note I said 'crossed controls', which is what Murray says [the instructor] corrected the bank with right on the yoke....

I can't see how the rudder vector can come into play in that axis if the wings are kept level with crossed controls. With (significant amounts:eek: ) of yaw-induced roll, yes, but not in the wings level situation. Which points towards the simple drag explanation, or one of the Scientist's more complex explanations.

cheers!
foggy.

[Croqueteer]

OK.